Official places the numbers come from

NZ Police: Proceedings (offender demographics)

Unique ethnicity, age and offence group for people proceeded against.

https://www.police.govt.nz/about-us/publications-statistics/data-and-statistics/policedatanz/proceedings-offender-demographics

NZ Police: policedata.nz

Recorded crime, resolution and district extracts.

https://www.police.govt.nz/about-us/publications-statistics/data-and-statistics/policedatanz

Ministry of Justice: children and young people in court

Finalised charges for children and young people, including Youth Court.

https://www.justice.govt.nz/justice-sector-policy/research-data/justice-statistics/data-tables/

Stats NZ: crime and justice

National collections and youth prosecution series.

https://www.stats.govt.nz/topics/crime-and-justice/

Stats NZ: census / estimated resident population

Ethnicity and age structure used for population shares.

https://www.stats.govt.nz/

Youth Court: youth justice statistics

Pointers into the official court tables.

https://www.youthcourt.govt.nz/youth-justice/youth-justice-statistics/